The objective of this invention is to disclose the process variables which are important in formation of a high impact polystyrene useful in making extruded sheet with balanced properties suitable for deep draw thermoforming of packaging containers.
Packaging containers are made of polystyrene because of its ability to be thermoformed readily on automatic machinery. The finished containers exhibit high stiffness even in thin walls because of the stiffness of polystyrene. Such containers are often over-printed, and polystyrene's ready acceptance of printing is useful in this application. These containers must possess good impact strength in order to contain the material between filling and use and to withstand the abuse normally encountered in storage and handling from room temperature to below room temperature. Such containers are often fabricated with a depth of draw at least as great as the diameter of the container or even greater.
A polystyrene resin for this application must possess impact strength in the finished deep drawn container. It is important for such a packaging grade high impact polystyrene that it be capable of producing extruded sheet with balanced impact and elongation properties. The sheet material possessing these balanced properties in transverse and across machine extrusion direction must also thermoform uniformly into deep drawn containers showing good impact resistance with no weakness in flex and impact. The container must resist splitting parallel to the axis of the container.
A high impact polystyrene for this application must also form sheet with sufficient melt strength to be handled readily on automatic thermoforming equipment which heats wide sheet webs and thermoforms the web into a plurality of deep drawn containers. These webs must not unduly sag under heaters and must still be thermoformed readily without tearing of the sheet during hot stretching. Also, it is advantageous to have impact polystyrene resins that with minor changes in the polymerization recipe for the resin can be used for injection molding and extrusion or extrusion alone.